In recent years, as mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries, which can be charged and discharged, as an energy source for the mobile devices has also sharply increased. As a result, much research has been carried out into a secondary battery that is capable of satisfying the wide variety of demands. In addition, the secondary battery has also attracted considerable attention as a power source for electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (Plug-in HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles, which use fossil fuels.
Consequently, electric vehicles (EV), which can be driven using only a battery, and hybrid electric vehicles (HEV), which uses both a battery and an engine, have been developed, and some of the electric vehicles (EV) and the hybrid electric vehicles (HEV) have now been commercialized. Nickel-metal hydride (Ni-MH) secondary batteries have been mainly used as a power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). In recent years, on the other hand, much research has also been carried out into lithium secondary batteries having high energy density, discharge voltage, and output stability, and some of the lithium secondary batteries have now been commercialized.
In order for the secondary battery to be used as a power source of a device, such as an electric vehicle (EV) or a hybrid electric vehicle (HEV), or a system that has various driving states, it is necessary for the secondary battery to have various output and capacity properties corresponding to the driving states of the device or the system. To this end, a hybrid-type battery pack, which is configured to have a structure in which a plurality of high-output, low-capacity secondary batteries and a plurality of low-output, high-capacity secondary batteries are included as unit cells, and in which the unit cells are connected to each other in series and/or in parallel, has been proposed.
FIG. 1 is a typical view showing an electrode assembly that constitutes a high-output, low-capacity secondary battery, which is received in the above-described hybrid-type battery pack.
Referring to FIG. 1, an electrode assembly 100 that constitutes a high-output, low-capacity secondary battery includes a positive electrode 110, which includes a positive electrode current collector 111 and high-output, low-capacity positive electrode materials 112 and 113 added to opposite main surfaces of the positive electrode current collector 111, a negative electrode 120, which includes a negative electrode current collector 121 and high-output, low-capacity negative electrode materials 122 and 123 added to opposite main surfaces of the negative electrode current collector 121, and a separator 130 interposed between the positive electrode 110 and the negative electrode 120. As a result, the electrode assembly 100 exhibits high-output, low-capacity properties.
FIG. 2 is a typical view showing an electrode assembly that constitutes a low-output, high-capacity secondary battery, which is received in the above-described hybrid-type battery pack.
Referring to FIG. 2, an electrode assembly 200 that constitutes a low-output, high-capacity secondary battery includes a positive electrode 210, which includes a positive electrode current collector 211 and low-output, high-capacity positive electrode materials 212 and 213 added to opposite main surfaces of the positive electrode current collector 211, a negative electrode 220, which includes a negative electrode current collector 221 and low-output, high-capacity negative electrode materials 222 and 223 added to opposite main surfaces of the negative electrode current collector 221, and a separator 230 interposed between the positive electrode 210 and the negative electrode 220. As a result, the electrode assembly 200 exhibits low-output, high-capacity properties.
That is, the conventional hybrid-type battery pack includes a secondary battery that exhibits high-output, low-capacity properties and a secondary battery that exhibits low-output, high-capacity properties such that the battery pack can operate appropriately for various driving states of a device that uses the battery pack as a power source.
FIG. 3 is a typical view showing a hybrid-type battery pack including a plurality of secondary batteries having different outputs and capacities.
Referring to FIG. 3, a hybrid-type battery pack 300 is configured to have a structure in which high-output, low-capacity secondary batteries 302 and low-output, high-capacity secondary batteries 303 are mounted in a pack case 301 as unit cells such that the secondary batteries operate independently depending upon the output and capacity properties thereof, and in which the secondary batteries are connected to each other via electrode terminal connection parts 304.
In a device that uses the battery pack 300, however, an initial driving condition is generally different from other driving conditions. The secondary batteries 302 and 303, which constitute the battery pack 300, have different self-discharge rates. As a result, the remaining capacities of the secondary batteries 302 and 303 may vary over time.
In order to overcome the imbalance in capacity between the secondary batteries having different outputs and capacities and to improve the safety, lifespan, output, and capacity properties of the secondary batteries, therefore, cell balancing current 305 flows between the high-output, low-capacity secondary batteries 302 and the low-output, high-capacity secondary batteries 303 such that voltages of the secondary batteries are made uniform.
Since the secondary batteries 302 and 303 are connected to each other in series and/or in parallel as unit cells via electrode terminals in an independent state depending upon the output and capacity properties of the battery pack, however, the cell balancing current 305 flows through the electrode terminal connection parts 304, via which the secondary batteries 302 and 303 are connected to each other. In this case, heat is generated at the electrode terminal connection parts 304 due to high resistance when the cell balancing current 305 continuously flows through the electrode terminal connection parts 304. If the heat is not effectively dissipated, but is allowed to accumulate, the batteries may be deteriorated, with the result that the durability and safety of the secondary batteries may be greatly reduced.
Therefore, there is a high necessity for technology that is capable of fundamentally solving the above problems.